Oral sex is surprisingly rare in the animal kingdom. Humans do it, of course. As do bonobos, our close relatives. But now researchers have observed the practice for the first time in a non-primate. During intercourse, female short-nosed fruit bats lick the genitals of their partner, a possible ploy to increase copulation time. The discovery suggests there may be a biological advantage to fellatio.

Senior author Libiao Zhang, a biologist at Guangdong Entomological Institute in Guangzhou, China, and colleagues had been studying the mating behavior of the short-nosed fruit bat (Cynopterus sphinx), which is native to southeast Asia. The researchers paired males and females in cages designed to mimic their natural environment, while infrared cameras beamed video of the creatures' nighttime liaisons back to the lab.

Most of what Zhang and colleagues saw didn't surprise them. The males built tents out of Chinese fan-palm leaves to attract the females. And both sexes groomed each other during courtship. But then came the shocker: After the male mounted the female from behind, she bent over and began licking his penis.

Of the 20 observed mating bat pairs, 70% of the females performed fellatio on the males, the team reports online this week in PLoS ONE. The males never withdrew while being licked, and the authors found that the longer a female licked, the longer copulation lasted (for each second of licking, the female bats gained 6 seconds of copulation). The team speculates that licking helps maintain the male's erection, and that the saliva increases lubrication, both of which may prolong intercourse. In all, fellating females mated for an average of 4 minutes, twice as long as the other females.

Zhang and colleagues have a few theories as to why the females would want to increase the length of intercourse. One idea is that it may facilitate sperm transport. Or it could keep males occupied--and thus away from rival females.

Fellatio may also offer protection against sexually transmitted diseases, based on the antimicrobial properties of saliva. Many male animals, including short-nosed fruit bats, lick their genitals after copulation, and in some species this has been shown to reduce the incidence of such diseases.

"The finding of fellatio in bats is exciting news," says Frans de Waal, a primatologist at Emory University in Atlanta who has worked extensively with bonobos. He says that although the behavior is likely rare, it may be more common than we think. "Part of the reason fellatio is rarely mentioned is shyness about this issue." The observation provides a unique opportunity to test some theories about the evolutionary role of fellatio, adds Paul Vasey, a behavioral scientist at the University of Lethbridge in Alberta, Canada. Although it's possible, he says, that bats are just being sexually playful, like their human and bonobo counterparts.